Method for treating a web

ABSTRACT

AN IMPROVED METHOD FOR SUBSTANTIALLY ELIMINATING CHARGE FROM A SURFACE OF A CORONA-TREATED WEB, PARTICULARLY FOR ENHANCING THE SURFACE&#39;&#39;S COATABILITY. THE IMPROVED METHOD EMPLOYS THE TECHNIQUE OF &#34;FLOODING&#34; THE SURFACE WITH CHARGE OF A FIRST POLARITY SO AS TO GENERALLY UNIFORMLY CHARGE THE SURFACE AND THEREAFTER REMOVING THE CHARGE IMPARTED TO THE SURFACE, SO AS TO LEAVE THE SURFACE GENERALLY FREE OF CHARGE. TO ENSURE POSITIVE CONTROL BETWEEN THE AMOUNT OF CHARGE ADDED TO THE SURFACE AND THE AMOUNT REMOVED, THE INVENTION PROVIDES THE IMPROVEMENT OF MONITORING THE SURFACE OF THE WEB FOR CHARGE VARIATION AND FOR NTE CHARGE. THE AMOUNT OF CHARGE ADDED TO AND/OR THE AMOUNT OF CHARGE REMOVED FROM THE SURFACE MAY BE SO CONTROLLED THAT THE CHARGE VARIATION AND THE NET CHARGE ON THE SURFACE IS LOWERED TO AN ACCEPTABLE LEVEL, PREFERABLY APPROXIMATELY ZERO.

Mny 1,'1973 w. c. KERR METHOD FOR TREATING A WEB Filed July so, 1971 o w .NDOR DQRDQSQ e mm 3 INVENTOR. 7% my A fro/m5) United States Patent 3,730,753 METHOD FOR TREATING A WEB William C. Kerr, Middlesex, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y. Filed July 30, 1971, Ser. No. 167,758 Int. Cl. B44d 1/092 US. Cl. 117-34 19 Claims ABSTRACT OF THE DISCLOSURE An improved method for substantially eliminating charge from a surface of a corona-treated web, particularly for enhancing the surfaces coatability. The improved method employs the technique of flooding the surface with charge of a first polarity so as to generally uniformly charge the surface and thereafter removing the charge imparted to the surface, so as to leave the surface generally free of charge. To ensure positive control between the amount of charge added to the surface and the amount removed, the invention provides the improvement of monitoring the surface of the web for charge variation and for net charge. The amount of charge added to and/or the amount of charge removed from the surface may be so controlled that the charge variation and the net charge on the surface is lowered to an acceptable level, preferably approximately zero.

BACKGROUND OF THE INVENTION This invention relates to the treatment of webs. More particularly, the invention relates to a method for treating a web so as to improve the webs coatability.

In the prior art, it is common practice to corona-treat a surface of a web to improve the surfaces receptivity to inks, adhesives, photographic emulsions, subbing layers, etc. For example, a web of photographic paper may be polyethylene-coated; and such coating may be coronatreated before the application of a photographic emulsion thereon. While corona-treatment of the polyethylene surface may enhance the receptivity of the surface to the emulsion, a problem arises in that corona-treatment of the surface usually leaves a non-uniform distribution of, i.e., a pattern of, electrostatic charge on the surface. In coating systems where the coating operation is delicate, i.e., easily disturbed, this charge pattern must be substantially removed before the web is introduced into the coating station, if uniform coatings are to be produced. Otherwise, the charge pattern will cause coating bead disturbances and thus cause the coating to be uneven.

Apparently, in the corona-treatment of the polyethylene surface by conventional alternating-current coronaproducing means, the alternating electric field created by the corona-producing means distributes charged particles of different polarities onto the surface in a non-uniform charge pattern. Thus, along the surface there may reside hills comprising a relatively large amount of positive charge, and adjacent thereto valleys of a relatively large amount of negative charge. As the emulsion coating is deposited on the web, it is afiected by the surface charge distribution, and in a sense the emulsion aligns with the charge pattern and forms what has been called crosslines, thus adversely affecting the photographic quality of the product in question.

A need has developed, therefore, to remove charge from a surface to be coated, or at least to reduce the charge, or the effect of its pattern, to an acceptable level before coating the surface with photographic emulsions.

The problem of removing the non-uniformly distributed charge from the corona-treated surface is believed to be complicated by the nature of such charge. The

charge may be comprised of two types:

3,730,753 Patented May 1., 1973 P CC (1) free charges on which the mating charges of opposite polarity are remote from the web, and

(2) polar or bound charges in which charges of opposite polarity are on the opposite side of the web.

Prior art methods of removing surface charges from the web work quite well when there are only free charges to be removed. However, polar or bound charges are extremely difficult to reduce to a satisfactory level.

One technique for removing the non-uniform distribution of charge on a corona-treated polymeric surface is described in United States Patent No. 3,531,314. In this patent a corona-treated web is advanced to a charging station, wherein a charged roller floods the hills and valleys of surface charge and imparts a highly uniform charge level to one surface of the web. Simultaneously, a counter electrode positioned on the other side of the web imparts a highly uniform charge of opposite polarity to the other side of the web. Thereafter, both sides may be passed over grounded rollers, or wound into rolls, to reduce the level of the charge so that a photographic or light-sensitive emulsion may be coated on one of the sides without substantial fogging of the emulsion. Obviously, winding of the web into rolls is undesirable where one wishes to corona-treat and coat in a continuous operation. The use of grounded rollers to dissipate the high level of charge imparted to both sides of the web while removing much of the charge still leaves a relatively high level of (bound) charge on the surface.

An improvement upon the technique recited in the afore-mentioned patent is described in United States application Ser. No. 804,470, filed on Mar. 5, 1969. This improvement employs the flooding technique disclosed in US. Pat. No. 3,531,314, but includes a series circuit feature wherein first and second electrodessuch as rollers-are connected to the oppositely charged terminals of a source of DC potential and are located on the face side of the web. Counter electrodes for the rollers are located on the other (hereinafter called the wire) side of the web; the counter electrodes being electrically connected to each other. In operation, at a first charging station the first roller applies, say, a large amount of positive charge to the face side of the web and induces its counter electrode to deposit an equal amount of negative charge onto the wire side of the web. The application of negative charge is able to neutralize the polar positive charge on the wire side, and thereby frees the polar negative charge on the face side which may then be either conducted away or neutralized by the positively charged roller. The web is then conveyed to a second charging station comprising a relatively negatively charged roller and its relatively positively charged counter electrode. At this point, positively charged particles are applied to the wire side of the web to neutralize the negative charge, thereby freeing the positive charge on the face side which may then be removed by the relatively negatively charged roller. One important feature of this technique is that because of the series circuit feature, the amount of charge applied to each surface at the second charging station is equal to the amount of charge of opposite polarity applied to each surface at the first charging station. However, with a web having a relatively conductive layer either as an external surface layer or a layer sandwiched by other layers, a problem arises in balancing the amount of charge that is applied to the web at the charging stations. Apparently, with such a web, the conductive layerwhich may be grounded through various rollers-causes the first roller to see or be exposed to ground and, as a result, some charge may leak to ground, causing the first roller to deposit more charge on the web than the second roller removes. To restore the balance between the charging stations, the

afore-mentioned application suggests reducing the potential difference between the firstsay, positively chargedroller and its counter electrode by positively biasing the counter electrode. While this technique works satisfactorily and may be capable of reducing the potential on the surface to be coated to acceptable levels, it cannot completely compensate for charge leakage where an extremely conductive grounded layer, such as aluminum, is on one surface of the web. Furthermore, the series circuit technique is expensive, since one large source of perhaps 40,000 volts and 4 ma. may be required. Still further, the use of two rollers to reduce the charge on the surface is less than what is otherwise desirable in that there is a tendency for the web to crease as it is conveyed about the rollers. A reduction in the number of rollers represents a highly desirable improvement.

SUMMARY OF THE INVENTION The invention provides an improved method for use in applying a coating to a corona-treated web surface. The invention employs the technique of substantially removing non-uniform charge patterns from the surface by flooding the web with charge of a first polarity and thereafter removing the charge from the web and improves over such technique, among other things, by sensing the web surface for net charge and charge variation so that the amount of charge flooded onto the surface may be assuredly or positively controlled relative to the amount of charge removed.

An object of the invention is to provide an improved method for treating a web surface to enhance the surfaces coatability.

Another object of the invention is to provide an improved method for removing charge patterns from a web surface for enhancing the surfaces coatability.

BRIEF DESCRIPTION OF THE DRAWING In FIG. 1, there is shown a representation of one apparatus for carrying out the improved method fully described herein.

In FIGS. 2 and 3, there are shown alternate embodiments for a portion of the apparatus of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With regard to the drawings, a coating apparatus is shown wherein a web of dielectric material or one having a dielectric coating is conveyed from an unwind station 13 to a corona-treatment station 15, hereafter referred to as Zone 1. As used herein, the term dielectric refers to any surface with a surface resistivity of about 10 ohms per square or greater. At Zone 1, the web 10 is conveyed about an electrically grounded metal roller 17 so that one web surface 12, is in intimate contact with the surface of the roller. On the opposite side of the web, web surface 11 is exposed to a conventional alternatingcurrent corona-treatment, so as to enhance its receptivity to a coating, such as, for example, inks, adhesives, photographic emulsions, subbing layers, etc. In coronatreatment of the surface, a glow of corona may be seen to extend from electrodes 21, 23, and 25 to the surface 11. The electrodes may be connected to a suitable alternating-current power supply 31 for providing a suitable level of potential for ionizing the air between the electrodes and the surface. As used herein, alternating-currents and voltages are any such currents and voltages which vary, such, for example, as sinusoidal and pulsating currents and voltages. If desired, provision may be made in Zone 1 for corona-treating web surface 12.

After corona-treatment, the dielectric surface becomes highly charged with charge that is, as indicated in the drawing, non-uniformly distributed along the surface 11. To remove this charge, which may be comprised of both polar and free charges, the web is conveyed to Zone 2. At Zone 2, the web 10 is wrapped about an electrically swu sd m a ro ler 2.7 with he W ar a s 12 in intimate contact with the conductive surface of the roller. The web surface 11 is exposed to negatively charged electrodes 29, 30 which flood a large amount of negatively charged particles onto the corona-treated surface. The electrodes may be electrically connected to the negative terminal of an adjustable 020,000 volt, 02 ma., source 33 of DC potential. The grounded roller 27 acts as a counter electrode for the electrodes 29, 30 and deposits a high positive charge onto the web surface 12.

As the web 10 is advanced along the roller 27, it moves beneath electrodes 37, 39 which may be electrically connected to the positive terminal of an adjustable 0- 20,000 volt, 0-2 ma., source 35 of DC potential. The elec trodes 37, 39 deposit a large amount of positively charged particles onto the web surface 11 to neutralize the negative charge previously imparted to the surface 11. The grounded roller 27 at this point functions as a counter electrode for the electrodes 37, 39; and as a source of negative potential, the roller neutralizes the positive charge on web surface 12. To remove any free charge on the web surface to be coated, a grounded electrode 41 may be positioned adjacent the web surface at a point where the web is no longer in contact with the grounded roller 27. Such a position, as shown in the drawing, may be just prior to the webs being conveyed about one of the grounded conveyance rollers 43. It should be appreciated that in Zone 2, no corona-treatment of the web need occur, nor is desirable. While a corona glow may appear in the vicinity of the electrodes 29, 30 and 37, 39, such glow need not extend to the surface 11, as it would during conventional corona-treatment processes.

In order to assuredly or positively control the charge level on surface 11, the web is conveyed about the ground ed roller 51 so that the web surface 12 is in intimate contact with the roller, the web surface 11 being exposed to an induction probe 53 of a fieldmeter. The probe 53 is electrically connected to the Input terminal of an electrometer 57, which amplifies the signal induced in the probe 53 by the charge on the web surface 11 and indicates the DC potential (i.e., net charge) of the surface 11. Connected to the Output terminal of the electrometer 57 is an AC voltmeter 59 which is capable of detecting a high frequency, i.e., AC components of, the potential on the surface 11, thereby to indicate that a variation in charge is present on the surface 11.

When an AC signal is detected by the AC voltmeter 59, it may be considered an indication that both sources 33 and 35 are not charging the web surface 11 to the extent necessary to eliminate the hills and valleys of charge imparted to surface 11 in Zone 1. By raising the potential of either or both sources 33, 35 through dials 67, 69 respectively, the non-uniform charge on the web surface 11 may be removed. With a uniform charge distribution on the surface 11, i.e., no substantial charge variation, the AC voltmeter will indicate zero or nearzero voltage. After the hills and valleys of charge have been removed from the web, the potentials of the sources 33 and 35 may be relatively varied to remove the uniform level of charge which may exist on the web. The electrometer 57 may be used to detect this uniform charge of DC potential and will also indicate its polarity. To reduce the uniform level of charge on the web surface 11, one need only adjust the sources 33, 35 until the electrometer indicates a zero or near-zero charge level.

Downstream of the probe 53 is a coating hopper 63 for applying a coating 65such as a photographic emulsionto the web surface 11. In the coating of a web with a photographic emulsion, the tolerable potential levels on the web surface 11 are preferably 50 volts DC and 50 volts AC, but there are instanceswhere higher potentials, say, volts AC and DC may be tolerated. In certain instances, potentials, say, of 20 volts AC and DC, may be required.

It should be appreciated that the grounded-roller counter electrode 27 may, if desired, be replaced by grounded electrodes 81, 82 (FIG. 2) or by electrodes 83, 84 (FIG. 3) that are connected to DC potential sources 85, 86, respectively. In operation, the alternate embodiments shown in FIGS. 2 and 3 can function in a manner which is similar to that described above for the operation of the Zone 2 apparatus of FIG. 1.

Other modifications may include the use of two probes, i.e., one placed adjacent the web between sources 33 and 35 and connected to an AC sensitive device, and the other placed as shown in FIG. 1 but connected to just a DC sensitive device. In this modification, the detection of an AC signal by the AC sensitive device may be considered an indication that the source 33 is not charging the web surface 11 to the extent necessary to remove the non-uniform charge pattern.

The invention has been described in relation to preferred embodiments wherein a web has received a non-uniform distribution of charge on its surface(s) during a corona-treatment process; however, in its broader aspects the invention is applicable to the removal of charge from webs which have been charged by other processes.

Furthermore, the invention in its broader aspects is not linked to particular web compositions nor to particular coatings to be placed thereon. The method for carrying out the invention may be quite useful in the treatment of webs comprised of films and surfaces of various materials, including polyolefins such as polyethylenes, polypropylenes, and ethylene-propylene copolymers, polystyrene, polybutenes, polypentenes, polyacrylic acid esters,

linear polyesters and polycarbonates such as polyethylene terephthalates, polyamides such as nylon, cellulose esters, polyacrylonitrile, polyvinylidene chloride and other copolymers of the indicated monomers, such as ethylenevinyl acetate copolymers. Coatings, other than the preferred light-sensitive silver halide gelatin emulsions may be placed on the web. Thus, non-light-sensitive gelatin emulsions, as well as other compositions which may be coated by a hopper such as coatings including magnetic oxides, organic solvents, etc., may be coated in accordance with the invention described herein.

The particular charge sensing devices are th'e preferred ones; however, other sensing devices may be used to either supplement or replace the preferred ones. For example, an oscilloscope may be added to or substituted for the AC or DC sensitive devices disclosed. Also, a slow-responding chart recorder may be used in conjunction with or as a replacement for the electrometer 57.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

I claim:

1. A process for the preparation of a surface of a web for deposition of a coating on the surface, the surface including an initial non-uniform distribution of electric charge that is to be substantially removed from the surface prior to deposition of the coating thereon, the process including the steps of:

(a) applying electric charge of a first polarity to the surface; and then (b) removing charge from the surface; and the improvement which comprises:

(c) sensing the net charge and the charge variation on the surface and relatively adjusting the amount of charge applies in step (a) and the amount of charge removed in step (b) so that the charge variation and the net charge on the surface are substantially reduced.

2. The invention according to claim 1 wherein in step (c), charge variation is sensed by sensing the alternating current potential on the surface after the surface has been treated in accordance with at least step (a).

3. The invention according to claim 2 wherein in step (0), net charge is sensed by sensing the direct current potential on the surface after the surface has been treated in accordance with at least steps (a) and (b).

4. The invention according to claim 3 wherein in steps (a) and (b), charge is being applied to and removed from the surface without corona-treating the surface.

5. The invention according to claim 4 wherein in step (a), charge of said first polarity is applied to said surface from a first DC potential charging means and in step (b) charge of opposite polarity to that of said first polarity is deposited from a second DC potential charging means to remove the charge applied in step (a).

6. In a process for the preparation of a surface of a web for deposition of a coating on the surface, the surface having an initial non-uniform distribution of electric charge that is to be substantially removed from the surface prior to coating, the process including the steps of:

(a) flooding the surface of the web with charge from a first DC potential charging means, which first charging means is adapted to apply to said surface a sufficient amount of electrical charge of a first polarity to uniformly electrically charge said surface with charge of said first polarity;

(b) depositing onto said surface after it has been treated in accordance with step (a) charge of a second polarity opposite in polarity to that applied in step (a) from a second DC potential charging means, which second charging means is adapted to deposit a sufficient amount of said charge of a second polarity to remove the charge applied to said surface in step (a), and the improvment which comprises;

(c) sensing the net charge on said surface after said surface has been treated in accordance with steps (a) and (b), and sensing the charge variation on said surface after said surface has been treated in accordance with at least step (a); and relatively adjusting the charging of and the removal of charge from said surface in steps (a) and (b) so that the charge variation on the surface is substantially removed and the net charge on the surface is substantially reduced.

7. The invention according to claim 6 wherein in steps (a) and (b) the surface is exposed to said first and second charging means without the surface being corona-treated by the charging means.

8. The invention of claim 6 wherein in step (c) charge variation is sensed by sensing the alternating current potential on the surface and net charge is sensed by sensing the direct current potential on said surface.

9. A process for coating a surface of a web, which process comprises the steps of (a) corona-treating the surface; and then (b) applying electric charge of a first polarity to the surface; and then (c) removing charge from the surface;

(d) coating the surface treated in accordance with steps (a) through (c);

and the improvement which comprises:

(e) sensing, prior to step (d), the net charge on the surface after the surface has been treated in accordance with steps (b) and (c), and sensing, prior to step (d), the charge variation on the surface after the surface has been treated in accordance with at least step (b); and relatively adjusting the amount of charge applied to and the amount of charge removed from the surface in steps (b) and (c) so that the charge ,variation on the surface is substantially removed and the net charge on the surface is substantially reduced.

10. The invention according to claim 9 wherein the web surface treated in accordance with steps (a) through (c) is sensed for charge variation and for net charge.

(c) is sensed for charge variation and for net charge.

11. The invention according to claim 9 wherein the surface to be coated is comprised of a dielectric material and wherein the surface is corona-treated in step (a) by an alternating current corona charging means.

12. The invention according to claim 11 wherein in step (d) the coating is delivered to the surface by a hopper.

13. The invention according to claim 12 wherein the coating comprises a light-sensitive photographic emulsion.

14. The invention according to claim wherein in step (e) charge variation is sensed by sensing the alternating current potential on the surface and net charge is sensed by sensing the direct-current potential on the surface.

15. A process for coating a dielectric surface of a web, which process comprises the steps of:

(a) corona-treating the surface to be coated with an AC corona charging means, so as to improve the adhesion of said surface to said coating; thereafter (b) conveying the web through a DC electric field of a first polarity so that charge of one polarity is applied to one surface of the web and charge of an opposite polarity is applied to the opposite surface of the web; thereafter (c) conveying the web through a second DC electric field that is of an opposite polarity to that in step (b) so as to remove the charges applied to the surfaces in step (b); thereafter ((1) coating the AC corona treated surface of said web after the web has been treated in accordance with steps (a) through (c);

and the improvement which comprises:

(e) sensing, prior to step (d), the net charge on the surface to be coated after the surface to be coated has been treated in accordance with steps (b) and (c), and sensing, prior to step (d), the charge variation on the surface to be coated after the surface to be coated has been treated in accordance with at least step (b); and relatively adjusting the charging of and the removal of charge from the surface to be coated in steps (b) and (0) so that the charge variation on the surface to be coated is substantially removed and the net charge on the surface to be coated is substantially reduced.

16. The invention according to claim 15 wherein the web is conveyed through the DC electric fields in steps (b) and (c) without being corona treated in the fields.

17. The invention according to claim 15 wherein in step (e) charge variation is sensed by sensing the alternating current potential on the surface and net charge is sensed by sensing the direct-current potential on the surface.

18. The invention according to claim 15 wherein in step (d) the coating is delivered to the surface by a hopper.

19. The invention according to claim 18 wherein the coating comprises a light-sensitive photographic emulsion.

References Cited UNITED STATES PATENTS 2,952,559 9/1960 Nadeau 11734 3,475,652 10/1969 Levy 317-2 F 3,531,314 9/1970 Kerr et al 11793.l CD 3,619,615 11/1971 Fish 324-32 WILLIAM D. MARTIN, Primary Examiner J. H. NEWSOME, Assistant Examiner U.S. Cl. X.R.

ill747 R, 47 A, 93.1 CD; 204168; 3 17-2 R, 2 A; 324-332, 72 

